Cube corner retroreflective sheeting having channels

ABSTRACT

The invention relates to a method of making retroreflective sheeting and other articles prepared from casting a moldable synthetic resin onto a tool having a microstructured surface.

RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 12/261,919,filed Oct. 30, 2008 issued as U.S. Pat. No. 7,744,228; which is adivision of U.S. application Ser. No. 11/869,776, filed Oct. 10, 2007,issued as U.S. Pat. No. 7,458,694; which is a continuation of U.S.application Ser. No. 11/080,694, filed Mar. 14, 2005, issued as U.S.Pat. No. 7,309,135; which is a division of U.S. application Ser. No.10/404,235, filed Apr. 1, 2003, issued as U.S. Pat. No. 6,884,371; whichclaims benefit from and priority to U.S. Provisional Application No.60/452,605, filed Mar. 6, 2003, the disclosures of which areincorporated by reference in their entirety herein.

FIELD OF THE INVENTION

The invention relates to a method of making retroreflective sheetingprepared from casting a moldable synthetic resin onto a tool having amicrostructured surface.

BACKGROUND OF THE INVENTION

Retroreflective materials are characterized by the ability to redirectlight incident on the material back toward the originating light source.This property has led to the widespread use of retroreflective sheetingfor a variety of traffic and personal safety uses. Retroreflectivesheeting is commonly employed in a variety of articles, for example,road signs, barricades, license plates, pavement markers and markingtape, as well as retroreflective tapes for vehicles and clothing.

Two known types of retroreflective sheeting are microsphere-basedsheeting and cube corner sheeting. Microsphere-based sheeting, sometimesreferred to as “beaded” sheeting, employs a multitude of microspherestypically at least partially embedded in a binder layer and havingassociated specular or diffuse reflecting materials (e.g., pigmentparticles, metal flakes or vapor coats, etc.) to retroreflect incidentlight. Cube corner retroreflective sheeting typically comprises a thintransparent layer having a substantially planar front surface and a rearstructured surface comprising a plurality of geometric structures, someor all of which include three reflective faces configured as a cubecorner element.

Cube corner retroreflective sheeting is commonly produced by firstmanufacturing a master mold that has a structured surface, suchstructured surface corresponding either to the desired cube cornerelement geometry in the finished sheeting or to a negative (inverted)copy thereof, depending upon whether the finished sheeting is to havecube corner pyramids or cube corner cavities (or both). Known methodsfor manufacturing the master mold include pin-bundling techniques,direct machining techniques, and techniques that employ laminae.

In pin bundling techniques, a plurality of pins, each having a geometricshape such as a cube corner element on one end, are assembled togetherto form a master mold. U.S. Pat. Nos. 1,591,572 (Stimson) and 3,926,402(Heenan) provide illustrative examples.

In direct machining techniques, a series of grooves are formed in thesurface of a planar substrate (e.g., metal plate) to form a master moldcomprising truncated cube corner elements. In one well known technique,three sets of parallel grooves intersect each other at 60 degreeincluded angles to form an array of cube corner elements, each having anequilateral base triangle (see U.S. Pat. No. 3,712,706 (Stamm)). Inanother technique, two sets of grooves intersect each other at an anglegreater than 60 degrees and a third set of grooves intersects each ofthe other two sets at an angle less than 60 degrees to form an array ofcanted cube corner element matched pairs (see U.S. Pat. No. 4,588,258(Hoopman)). In direct machining, a large number of individual faces aretypically formed along the same groove formed by continuous motion of acutting tool. Thus, such individual faces maintain their alignmentthroughout the mold fabrication procedure. For this reason, directmachining techniques offer the ability to accurately machine very smallcube corner elements. A drawback to direct machining techniques,however, has been reduced design flexibility in the types of cube cornergeometries that can be produced, which in turn affects the total lightreturn.

In techniques that employ laminae, a plurality of thin sheets (i.e.,plates) referred to as laminae having geometric shapes formed on onelongitudinal edge are assembled to form a master mold. Lamina techniquesare generally less labor intensive than pin bundling techniques becausefewer parts are separately machined. For example, one lamina typicallycomprises about 400-1000 individual cube corner elements in comparisonto each pin comprising a single cube corner element. Illustrativeexamples of lamina techniques can be found in EP 0 844 056 A1 (Mimura);U.S. Pat. No. 6,015,214 (Heenan); U.S. Pat. No. 5,981,032 (Smith); U.S.Pat. No. 6,159,407 (Krinke) and U.S. Pat. No. 6,257,860 (Luttrell).

The base edges of adjacent cube corner elements of truncated cube cornerarrays are typically coplanar. Other cube corner element structures,described as “full cubes” or “preferred geometry (PG) cube cornerelements” typically comprise at least two non-dihedral edges that arenot coplanar. Such structures typically exhibit a higher total lightreturn in comparison to truncated cube corner elements. Certain PG cubecorner elements may be fabricated via direct machining of a sequence ofsubstrates, as described in WO 00/60385. However, it is difficult tomaintain geometric accuracy with this multi-step fabrication process.Design constraints may also be evident in the resulting PG cube cornerelements and/or arrangement of elements. By contrast, pin bundling andtechniques that employ laminae allow for the formation of a variety ofshapes and arrangements of PG cube corner elements. Unlike pin bundling,however, techniques that employ laminae also advantageously provide theability to form relatively smaller PG cube corner elements.

After manufacturing a master mold the master mold is typicallyreplicated using any suitable technique such as conventional nickelelectroforming to produce a tool of a desired size for formingmicrostructured sheeting. Multigenerational positive and negative copytools are thus formed, such tools having substantially the same degreeof precise cube formation as the master. Electroforming techniques suchas described in U.S. Pat. Nos. 4,478,769 (Pricone et al.) and 5,156,863(Pricone) as well as U.S. Pat. No. 6,159,407 (Krinke) are known. Aplurality of replications are often joined together for example bywelding such as described in U.S. Pat. No. 6,322,652 (Paulson). Theresulting tooling may then be employed for forming cube cornerretroreflective sheeting by processes such as embossing, extruding, orcast-and-curing, as known in the art.

For example, U.S. Pat. Nos. 3,684,348 (Rowland) and 3,811,983 (Rowland)describe retroreflective material and a method of making a compositematerial wherein a fluid molding material is deposited on a moldingsurface having cube corner recesses and a preformed body member appliedthereto. The molding material is then hardened and bonded to the bodymember. The molding material may be a molten resin and thesolidification thereof accomplished at least in part by cooling, theinherent nature of the molten resin producing bonding to the body memberthereof. Alternatively, the molding material may be fluid resin havingcross-linkable groups and the solidification thereof may be accomplishedat least in part by cross-linking of the resin. The molding material mayalso be a partially polymerized resin formulation and wherein thesolidification thereof is accomplished at least in part bypolymerization of the resin formulation.

Various retroreflective sheeting comprising truncated cube corner arrayshave been commercially successful such as retroreflective sheetingcommercially available from 3M Company (“3M”), St. Paul, Minn. under thetrade designation “3M Scotchlite Brand Reflective Sheeting 3990 VIP”.Although described in the patent literature, retroreflective sheetingcomprising an array of full cubes or PG cube corner elements has notbeen manufactured commercially or sold. In order to accommodate thecommercial success of retroreflective sheeting comprising an array offull cubes or PG cube corner elements, industry would find advantage inimproved methods of making retroreflective sheeting comprising sucharrays.

SUMMARY OF THE INVENTION

A characteristic of certain tools for making retroreflective sheetingcomprising an array of full cube or PG cube corner cavities is thepresence of channels. The present inventor has found that the presenceof such channel as well as the orientation of such channels relative tothe direction of relative motion of the tool in comparison to the resindelivery system (e.g., the advancing tool) during manufacture ofsheeting has a significant effect on the quality of the replication aswell as the rate of manufacturing the sheeting. The present inventionrelates to a method of making retroreflective sheeting providing a toolcomprising a cube corner microstructured surface with at least onechannel, advancing the tool in a direction such that the channel issubstantially parallel to the direction of the advancing tool, casting amoldable resin onto said tool surface, solidifying the resin forming aretroreflective sheet having a surface comprising cube corner elements;and removing the sheet from the tool.

In another embodiment, the invention discloses retroreflective sheetingcomprising a pair of longitudinal peripheral edges and at least one rowof PG cube corner microstructures and at least one channel extendingsubstantially parallel to the row; wherein the channel is substantiallyparallel to the longitudinal peripheral edges of the sheeting. Thelongitudinal peripheral edges span the sheeting in its maximum dimensionas manufactured. The sheeting is preferably provided in a roll-good.

In each of these embodiments the cube corner microstructures arepreferably PG cube corner microstructures. The cube cornermicrostructures may be cavities or elements. The channel may be aprimary groove channel comprising a first planar face and second planarface intersecting at a common vertex. Alternatively or in additionthereto, the channel may be a structured channel comprising a first facecomprising cube corner faces and second face comprising opposing cubecorner faces. Alternatively, the structured channel may comprise theintersection of opposing non-dihedral edges of opposing cube cornermicrostructures. Alternatively, or in addition thereto, the channel maybe a cube cavity channel comprising a first planar face and a secondstructured face. The first planar face is preferably a replica of aprimary groove face. A replica of a cube cavity channel provides cubecorner elements. The resin may be a thermoplastic resin provided moltenor provided in the form of sheet. The (e.g., thermosetting, radiationcurable) resin may optionally be provided on a carrier web.

BRIEF DESCRIPTION OF THE DRAWINGS

In the several figures of the attached drawing, like parts bear likereference numerals, and:

FIG. 1 is a perspective view of an exemplary single lamina prior toformation of cube corner microstructures.

FIG. 2 is a perspective view of a master tool comprising four laminaecomprising cube corner elements microstructures.

FIG. 3 is a perspective view of a tool that is a replica of the mastertool of FIG. 2 comprising cube corner cavity microstructures.

FIG. 4 a is a side view of an exemplary method of extruding moltenpolymeric resin onto a tool with a slot die according to the presentinvention.

FIG. 4 b is an enlarged view of the tool.

FIG. 4 c is an enlarged view of the resin on the tool.

FIG. 5 is a side view of an exemplary slot die apparatus for use in themethod of the invention.

FIG. 6 depicts a detailed side view of an exemplary slot die apparatusfor use in the present invention.

FIGS. 7 a-7 d depict photographs of retroreflective sheeting preparedwith an exemplary method and exemplary slot die apparatus of theinvention.

FIG. 8 depicts retroreflective sheeting prepared from tooling havingdown-web channels in comparison to crossweb channels.

FIG. 9 a depicts a photograph of retroreflective sheeting manufacturedwherein the channels were oriented downweb (i.e., parallel) relative tothe direction of motion of the tool.

FIG. 9 b depicts a photograph of retroreflective sheeting manufacturedwherein the channels were orientated crossweb (i.e., perpendicular)relative to the direction of motion of the tool.

FIG. 10 depicts a top plan view of a lamina having skewed side grooves.

FIG. 11 depicts each of the dihedral angles of a cube corner element.

FIG. 12 depicts a side view of a cube corner element of a laminadepicting positive and negative inclination.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method and apparatus of the invention relate to makingmicrostructured sheeting articles such as retroreflective sheeting.

As used herein, “sheeting” refers to a thin piece of polymeric (e.g.,synthetic) material. The sheeting may be of any width and length, suchdimension only being limited by the equipment (e.g., width of the tool,width of the slot die orifice, etc.) from which the sheeting was made.The thickness of retroreflective sheeting typically ranges from about0.004 inches (0.1016 mm) to about 0.10 inches (2.54 mm). Preferably thethickness of retroreflective sheeting is less than about 0.020 inches(0.508 mm) and more preferably less than about 0.014 inches (0.3556 mm).In the case of retroreflective sheeting, the width is typically at least30 inches (122 cm) and preferably at least 48 inches (76 cm). Thesheeting is typically continuous in its length for up to about 50 yards(45.5 m) to 100 yards (91 m) such that the sheeting is provided in aconveniently handled roll-good. Additional layers such as seal films oroverlays may also be utilized. Alternatively, however, the sheeting maybe manufactured as individual sheets rather than as a roll-good. In suchembodiments, the sheets preferably correspond in dimensions to thefinished article. For example, the retroreflective sheeting, may havethe dimensions of a standard U.S. sign (e.g., 30 inches by 30 inches (76cm by 76 cm)) and thus the microstructured tool employed to prepare thesheeting may have about the same dimensions. Smaller articles such aslicense plates or reflective buttons may employ sheeting having acorrespondingly smaller dimension.

Regardless of whether the retroreflective sheeting is provided as aroll-good or as a sheet, the sheeting comprises a pair of longitudinalperipheral edges such as depicted by 2 a and 2 b of FIG. 8. Suchlongitudinal peripheral edges typically span the sheeting in a maximumdirection. Further the longitudinal peripheral edges are parallel withthe direction of motion of the advancing tool and/or advancing moldableresin from the process in which the sheeting was made. Preferably, rowsof PG cube corner elements are aligned parallel to these longitudinalperipheral edges.

As used herein, “microstructured” refers to at least one major surfaceof the sheeting comprising structures having a lateral dimension (e.g.,distance between groove vertices of the cube corner structures) of lessthan 0.25 inches (6.35 mm), preferably less than 0.125 inches (3,175 mm)and more preferably less than 0.04 inches (1 mm). The lateral dimension,particularly of cube corner elements, is preferably less than 0.020inches (0.508 mm) and more preferably less than 0.007 inches (0.1778mm). The microstructures have an average height ranging from about 0.001inches (0.0254 mm) to 0.010 inches (0.254 mm), with a height of lessthan 0.004 inches (0.1016 mm) being most typical. Further, the smallestlateral dimension of a cube corner microstructure us typically at least0.0005 inches (0.0127 mm). Cube corner microstructures may compriseeither cube corner cavities or, preferably, cube corner elements havingpeaks.

As used herein, “casting” refers to forming a moldable resin into asheet having a microstructured surface by contacting the moldable resinwith a microstructured mold surface. The moldable resin is preferablysufficiently fluid such that it may be extruded, pumped or poured onto amolding tool having the microstructured surface. The viscosity of theresin may vary widely. Polymerizable resins are often low to moderateviscosity liquids, whereas thermoplastic resins may be relativelyviscous at the casting temperature. Alternatively the moldable resin maybe provided in the form of a sheet that is contacted with an advancingembossing tool or by rolling bank processes that involve the contactinga coated carrier web with a tool.

The tool used herein is typically obtained by first manufacturing amaster mold that has a microstructured surface. Method of manufacturingmaster molds are known. Master molds employed for making retroreflectivesheeting are typically prepared from pin-bundling techniques, directmachining techniques, and techniques that employ laminae, as describedin the art. The master mold for use in the invention is preferablyderived from a laminae technique.

With reference to FIG. 1, lamina 10 includes a first major surface 12and an opposing second major surface (not shown). Lamina 10 furtherincludes working surface 16 and an opposing bottom surface extendingbetween first major surface 12 and second major surface. Lamina 10further includes a first end surface 20 and an opposing second endsurface 22.

Lamina 10 can be characterized in three-dimensional space with the samesuperimposed Cartesian coordinate system. A first reference plane 24 iscentered between major surfaces 12. First reference plane 24, referredto as the x-z plane, has the y-axis as its normal vector. A secondreference plane 26, referred to as the x-y plane, extends substantiallycoplanar with working surface 16 of lamina 10 and has the z-axis as itsnormal vector. A third reference plane 28, referred to as the y-z plane,is centered between first end surface 20 and second end surface 22 andhas the x-axis as its normal vector.

In the method of machining lamina comprising cube corner microstructuresa first groove set, an optional second groove set, and preferably athird primary groove are formed with a groove-forming machine. As usedherein, the term “groove set” refers to grooves formed in workingsurface 16 of the lamina 10 that range from being nominally parallel tonon-parallel to within 1° to the adjacent grooves in the groove set.Alternatively or in addition thereto the grooves of the groove set mayrange from being nominally parallel to non-parallel to within 1° toparticular reference planes as will subsequently be described.Accordingly, each characteristic with regard to an individual groove andor the groove of a groove set (e.g., perpendicular, angle. etc.) will beunderstood to have this same degree of potential deviation. Nominallyparallel grooves are grooves wherein no purposeful variation has beenintroduced within the degree of precision of the groove-forming machine.

In general, the first groove set comprises a plurality of grooves havingrespective groove vertices that intersect the first major surface 12 andworking surface 16 of lamina. Although working surface 16 may include aportion that remains unaltered (i.e., unstructured), it is preferredthat working surface 16 is substantially free of unstructured surfaceportions.

The second groove set, (i.e., when present) comprises a plurality ofgrooves having respective groove vertices that intersect the first majorsurface and the working surface 16 of lamina. The first and secondgroove sets intersect approximately along a first reference plane 24 toform a structured surface including a plurality of alternating peaks andV-shaped valleys. Although not depicted, this embodiment may appear thesame as the combination of lamina 200 and lamina 300 in FIG. 2.

Both the first and second groove sets may also be referred to herein as“side grooves”. As used herein, side groove refers to an individualgroove or a groove set wherein the groove(s) range from being nominallyparallel to non-parallel within 1°, per their respective groovedirection vectors, to at least one adjacent groove and preferably to allthe grooves of the side groove set. The direction of a particular grooveis defined by a vector aligned with the groove vertex. The groovedirection vector may be defined by its components in the x, y, and zdirections, the x-axis being perpendicular to reference plane 28 andy-axis being perpendicular to reference plan 24. Alternatively or inaddition thereto, side groove refers to a groove that ranges from beingnominally parallel to reference plane 28 to nonparallel to referenceplane 28 to within 1°. Side grooves may optionally be perpendicular toreference plane 24 to this same degree of deviation.

After formation of the groove sets, working surface 16 ismicrostructured. As used herein, “microstructured” refers to at leastone major surface of the sheeting comprising structures having a lateraldimension (e.g., distance between groove vertices of the cube cornerstructures) of less than 0.25 inches (6.35 mm), preferably less than0.125 inches (3.175 mm) and more preferably less than 0.04 inches (1mm). The lateral dimension of cube corner elements, is preferably lessthan 0.020 inches (0.508 mm) and more preferably less than 0.007 inches(0.1778 mm). Accordingly, the respective groove vertices 33 and 41 arepreferably separated by this same distance throughout the groove otherthan the small variations resulting from non-parallel grooves. Themicrostructures have an average height ranging from about 0.001 inches(0.0254 mm) to 0.010 inches (0.254 mm), with a height of less than 0.004inches (0.1016 mm) being most typical. Further, the lateral dimension ofa cube corner microstructure is typically at least 0.0005 inches (0.0127mm). Cube corner microstructures may comprise either cube cornercavities or, preferably, cube corner elements having peaks.

The side grooves may comprise small purposeful variations for thepurpose of improving the retroreflected divergence profile such asincluded angle errors, and/or skew, and/or inclination. The advantagesof skew and/or inclination are described in U.S. patent application Ser.No. 60/452,464 filed Mar. 6, 2003; incorporated herein by reference.U.S. patent application Ser. No. 60/452,464 was filed concurrently withU.S. patent application Ser. No. 60/452,605, to which the presentapplication claims priority.

Skew and/or inclination provides cubes with a variety of controlleddihedral angle errors or multiple non-orthogonality (MNO) and thusimproves the divergence profile of the finished product. As used herein“skew” refers to the deviation from parallel with reference to referenceplane 28.

FIG. 10 shows an exaggerated example in plan view of a single laminawith one row of cube corner elements comprising skewed grooves. Sidegrooves 80 a and 80 b are cut with positive skew, grooves 80 c and 80 ewithout skew, and groove 80 d with negative skew. The path of the sidegrooves 80 has been extended in FIG. 10 for clarity. Provided sidegrooves 80 a, 80 c, and 80 e have the same included angle (e.g., 75°,first groove sub-set), the same cutting tool or diamond can be used toform all of these grooves. Further, if the alternating grooves, namely80 b and 80 d have the same included angle (e.g., 105°, second groovesub-set) 80 b and 80 d can be cut with a second diamond. The primarygroove face 50 may also be cut with one of these diamonds if the primarygroove half angle is sufficiently close to the side groove half anglefor the first or second sub-sets. Optionally, one of the cutting toolsmay be rotated during cutting of the primary groove face in order toachieve the correct primary groove half angle. The primary groove faceis preferably aligned with the side of the lamina. Thus, the entirelamina can be cut incorporating MNO with the use of only two diamonds.Within each groove set skew can be easily varied during machining toproduce a range of dihedral angles. Cube corners in general have threedihedral angles attributed to the intersections of the three cube faces.The deviation of these angles from 90° is commonly termed the dihedralangle error and may be designated by dihedral 1-2, dihedral 1-3, anddihedral 2-3. In one naming convention, as depicted in FIG. 11, cubedihedral angle 1-3 is formed by the intersection of groove surface 82with primary groove face 50, cube dihedral 1-2 is formed by theintersection of groove surface 84 with primary groove face 50, and cubedihedral 2-3 is formed by the intersection of groove surface 84 withgroove surface 82. For a given groove, positive skew (80 a, 80 b)results in decreasing dihedral 1-3 and increasing dihedral 1-2 whilenegative skew results in increasing dihedral 1-3 and decreasing dihedral1-2. For example, with reference to FIG. 10 four different cubes areformed. The first cube 86 a has groove surfaces (i.e., faces) 82 a and84 b, the second cube 86 b groove surfaces 82 b and 84 c, the third cube86 c groove surfaces 82 c and 84 d, and the fourth cube 86 d has groovesurfaces 82 d and 84 e. The intersection of groove surfaces 82 a, 82 b,and 84 d with groove face 50 are less than 90°, whereas the intersectionof groove surfaces 84 b and 82 d with groove face 50 are greater than90°. The intersection of groove surfaces 82 c, 84 c, and 84 e withgroove face 50 are 90° since grooves 80 c and 80 e are without skew. Thepreceding discussion assumes that the inclination (as will subsequentlybe defined) is the same for all the side grooves in FIG. 10 and equalszero. The (e.g., PG) cube corner elements are trapezoids orparallelograms (i.e., exclusive of rectangles) in plan view shape as aresult of using skewed grooves during machining.

Alternatively, or in addition to the features previously described, theside grooves may comprise positive or negative inclination.“Inclination” refers to the deviation in slope in reference plane 28 ofa particular side groove from the nominal orthogonal slope (i.e., theslope of the vector normal to the primary groove surface). The directionof a side groove is defined by a vector aligned with the vertex of saidgroove. Orthogonal slope is defined as the slope in which the vertex 90of a groove would be parallel to the normal vector of groove face 50(normal to groove face 50) for skew equal to zero. In one possiblenaming convention, positive inclination 92 results in decreasing bothdihedral 1-3 and dihedral 1-2 for a given side groove while negativeinclination 94 results in increasing both dihedral 1-3 and dihedral 1-2.

Combining skew and/or inclination during machining provides significantflexibility in varying the dihedral angle errors of the cube cornerelements on a given lamina. Such flexibility is independent of cant.Accordingly skew and/or inclination may be employed with uncanted cubes,forward canted cubes, backward canted cubes, as well as sideways cantedcubes. The use of skew and/or inclination provides a distinct advantageas it can be introduced during the machining of individual laminawithout changing the tool (e.g., diamond) used to cut the side grooves.This can significantly reduce machining time as it typically can takehours to correctly set angles after a tool change. Furthermore, dihedral1-2 and dihedral 1-3 may be varied in opposition using skew and/orinclination. “Varied in opposition” as used herein is defined asintentionally providing within a given cube corner on a lamina dihedral1-2 and 1-3 errors (differences from)90° that differ in magnitude by atleast 1 arc minute and/or sign more preferably by at least ½ arcminutes, and most preferably by at least ¼ arc minutes. Further, the(e.g., side) grooves may comprise a variety of different components ofskew and/or inclination along a single lamina.

The lamina preferably comprises a primary groove face that extendssubstantially the full length of the lamina. Formation of a primarygroove face results in a structured surface that includes a plurality ofcube corner elements having three perpendicular or approximatelyperpendicular optical faces on the lamina. Typically, the intersectionof such primary groove face with either working surface 12 or 14 isnominally parallel to reference plane 24 and 26. A single lamina mayhave a single primary groove face, a pair of groove faces on opposingsides and/or a primary groove along the intersection of working surface16 with reference plane 24 that concurrently provides a pair of primarygroove faces. A pair of single laminae with opposing orientations andpreferably multiple laminae (e.g., four lamina identified as 100, 200,300 and 400 in FIG. 2) with opposing orientations are typicallyassembled such that their respective primary groove faces form a primarygroove 52, for example as depicted with reference to FIG. 2.

The master mold is then replicated using any suitable technique such asconventional nickel electroforming to produce a tool of a desired sizefor forming cube corner retroreflective sheeting. Electroformingtechniques such as described in U.S. Pat. Nos. 4,478,769 (Pricone etal.) and 5,156,863 (Pricone); U.S. Pat. No. 6,159,407 (Krinke); whereasa preferred technique of forming grooves on lamina, assembling thelaminae, and replicating the microstructured surface is described inU.S. Pat. No. 7,174,619. A plurality of replications are often joinedtogether for example by welding such as described in U.S. Pat. No.6,322,652 (Paulson).

To form a master tool of suitable size for forming retroreflectivesheeting, a plurality of toolings (also referred to as tiles) are formedby electroplating the surface of the master tool to form negativecopies, subsequently electroplating the negative copies to form positivecopies, electroplating the positive copies to form a second generationnegative copies, etc. The positive copy has the same cube corner elementstructure as the master tool, whereas the negative copy is the cubecavity replica. Accordingly, a negative copy tool (e.g., FIG. 3) isemployed to make a positive copy (i.e., cube corner element) sheetingwhereas, a positive copy tool (e.g., FIG. 2) is employed to make anegative copy (i.e., cube corner cavity) sheeting. Tiling such toolingstogether can then assemble a master tool of the desired size. In thepresent invention the toolings are typically tiled in the sameorientation such that the channels are substantially continuous betweenjoined tooling portions.

Regardless of the manner is which the tool was derived and regardless ofwhether the tool comprises cube corner element microstructures whereinthe faces intersect at a peak or cube corner cavity microstructureswherein the replication thereof forms a cube corner element, the toolemployed in the method of the invention comprises at least one andtypically a plurality of channels. The channels are typically parallelto the rows of the cube corner microstructures (e.g., the cube cornermicrostructures formed on individual laminae).

In one aspect, the channel is defined by a primary groove. As depictedin FIG. 2, a primary groove channel 52 is typically created by a pair ofadjacent primary groove faces. Alternatively or in addition thereto, aprimary groove channel may be present along the intersection of workingsurface 16 with reference plane 24 per FIG. 1. The primary groove facestypically intersect forming a line. This intersection may also bereferred to as a vertex. Primary groove channels differ from groovesformed by direct machining of truncated cube corner arrays. In oneaspect, the vertex of any one direct-machined groove of truncated cubecorner arrays is typically intersected by grooves of other groove sets(e.g., formed at 60° to a first groove set) and groove faces are notcontinuous. In contrast, the bottom portion (e.g., vertex) of a primarygroove is typically not intersected by other grooves. Accordingly, thegroove faces of truncated cube corner arrays comprise a plurality ofopposing triangular faces, whereas primary groove faces comprisenon-triangular faces such as pentagons. In a truncated cube cornerarrays the tool comprises a plurality of elements or a plurality ofcavities, but not both in the same tool. In contrast, tools comprisingfull cube or PG cube corner arrays have a combination of cavities (e.g.,cube corner cavities and other cavities) and protrusions (e.g., cubecorner elements and other structures). Typically, at least about 10% toas much as about 50% of the faces of the primary groove are continuousand above the groove vertex line.

In another aspect, in alternative or in addition to the presence of aprimary groove channel(s), the tool may comprise a channel 54 andpreferably a plurality of channels formed by the intersection of a pairof rows of microstructured elements such as microstructured elementshaving opposing orientations as depicted in FIG. 2. Channels of thistype will be referred to as “structured channels” herein. Suchstructured channels may be present between primary groove channels.Further, such structured channels are typically parallel to the primarygroove channel. For example, the tool may have primary groove channelsalternated with structured channels as depicted in FIG. 2. Structuredchannels do not intersect at a linear line. Rather the intersectiontypically comprises a plurality of line segments formed by peaks andvalleys, such as peaks and valleys alternating in a repeating pattern.The faces of the structured channels preferably comprise cube cornerfaces (e.g., from adjacent rows of cubes formed on adjacent laminae).

Since the retroreflective sheeting is a replica of the tool, the tool ofFIG. 2 produces cube corner microstructures comprising cube cornercavities. Accordingly, retroreflective sheeting comprising cube cornerelement microstructures are derived from a tool comprising cube cornercavities as depicted in FIG. 3. The channels 53 of this type of tooldiffer from that of the channels of FIG. 2. In this embodiment, each ofthe channels generally comprises a substantially continuous planar face51. This face may preferably be derived from the replication of aprimary groove face. Alternatively, this face may result from thereplication of a portion of a major surface (e.g., 12 of FIG. 1).Alternatively, this face may be discontinuous wherein the face isinterrupted by structures intersecting with such face. The second faceof the channel is typically a structured face. In contrast to thestructured face depicted in FIG. 2, the structures of the second face ofFIG. 3 as depicted on the tool are not cube corner elements structures.However, the channel formed between planar face 51 and the secondstructured face forms a cube corner cavity, meaning that the replicationof the cavity forms cube corner elements (e.g., in a row). Thus, thesechannels may be referred to a cube corner cavity channels.

Channels can be further characterized with respect to a ratio of thedepth of a channel relative the overall height of the microstructuredsurface, i.e., distance along the z-axis between the highest and lowestpoints of the microstructures. Although this ratio can vary, the ratiois preferably greater than about 0.4 (e.g., 0.5, 0.6, 0.7, 0.8, 0.9,approaching 1) to obtain good replication at speeds of at least 10feet/min.

In general, there is typically at least one channel for each row ofelements. Each channels typically extends the entire length of a lamina,entire length of a row of cubes, the entire dimension of an array, orthe entire length of the sheeting. However, at minimum, the channelsextend for a length of at least about 10× its width, preferably at least100× its width and more preferably at least 500× its width. Regardlessof the type of channel, i.e., primary groove channel, structuredchannel, or cube corner cavity channel, the method of the inventionemploys providing the tool in a delivery system (e.g., dispensing devicesuch as a slot die apparatus) such that a major portion of such channelsand preferably substantially the totality of the channels aresubstantially parallel to the relative motion of the tool in comparisonto the delivery system, i.e., direction of the advancing tool and/oradvancing dispensing apparatus. Substantially parallel as it relates tothe direction or orientation of the channels relative to the directionof filling of the channel refers to the acute angle formed by these twodirections. Preferably, the orientation of the channel(s) does not varyfrom 0° by more than 20°, and more preferably by no more than 10° (e.g.,9°, 8°, 7°, 6°), and most preferably by no more than 5° (e.g., 5°, 4°,3°, 2°, 1°). The method of the invention is described with reference tothe use of a slot die apparatus as a dispensing means for providing themoldable (e.g., fluid) resin. As used herein, “slot die apparatus”refers to an apparatus comprising a cavity that includes a resindistribution portion, the arrangement of which can be of various designs(e.g., coat hanger, T-slot, etc.), wherein the cavity terminates in aslot orifice provided between a pair of die lips. The slot orifice istypically rectangular. Slot die apparatus are typically equipped withvarious other components such as adjusting bolts, electrical heaters,thermocouples, etc. as are known in the art. The dimensions of the slotorifice may vary. For example the width may vary from 0.010 inches to0.1 inches, whereas the length may vary from 2 inches to 60 inches(i.e., width of the coating line).

Other dispensing apparatus may alternatively be employed in place of theexemplary slot die apparatus described herein. For example one or moreneedle delivery system may be employed. Further the moldable resin mayalternatively be provided as a sheet that is contact with an embossingtool (i.e., at least one of which is advancing) or the moldable resinmay be provided on a carrier web such as in the case of rolling bankprocesses. Depending on the dispensing apparatus, the orifice may be inclose proximity to the tool surface or somewhat removed.

For example, the tool may be employed as an embossing tool to formretroreflective articles, such as described in U.S. Pat. No. 4,601,861(Pricone). Alternatively, the retroreflective sheeting can bemanufactured as a layered product by casting the cube-corner elementsagainst a preformed film as taught in PCT publication No. WO 95/11464and U.S. Pat. No. 3,684,348, or by laminating a preformed film topreformed cube-corner elements.

The method of making retroreflective sheeting via casting a hardenablefluid synthetic resin, in the absence of the invention described hereinis generally known from for example U.S. Pat. Nos. 3,811,983 (Rowland);3,689,346 (Rowland); and U.S. Pat. No. 5,961,846 (Benson Jr.).

With reference to FIGS. 4 a-4 c, a representative manufacturingapparatus and process 1010 includes advancing a tool 1200 having amicrostructured surface 1100, by means for example of drive rolls 1400 aand/or 1400 b; casting a fluid synthetic resin onto the microstructuredsurface of the tool with a slot die apparatus 1000; allowing the resinto sufficiently harden (i.e., solidify) while in contact with the toolforming a sheet 1600; and removing the sheet from the tool. In the caseof continuous production, the leading edge of the sheeting is removedfrom the tool surface with for example stripper roll 1800. The directionof filling of the tool is the direction of relative motion 310 of thetool relative to the delivery system (e.g., slot die apparatus).Accordingly, in FIGS. 4 a-4 c, the direction of filling is normal to theorifice of the dispensing apparatus.

Although the slot die apparatus and advancing tool are depicted in avertical arrangement, horizontal or other arrangements (i.e., anglesbetween horizontal and vertical) may also be employed. Regardless of theparticular arrangement, the slot die apparatus provides the fluid resinto the microstructured tool at the orifice of the slot die apparatus,preferably in a direction normal to the tool. In addition, themanufacturing process may include multiple slot die apparatusarrangements. For example, a first slot die apparatus may be provided topartially fill the cube cavities followed by a second slot die providedto fill the remainder of the cavity.

The die is mounted in a substantial mechanical framework that is capableof being moved towards the advancing tool surface by suitable means suchas jackscrews or hydraulic cylinders. Alternatively, the die may bestationary and the advancing tool surface moved towards the die. Whenthe die is about 0.020 inches from the tool, the fluid synthetic resin(e.g., molten thermoplastic polymeric material) contacts the toolforming a continuous layer of the resin on the microstructured toolsurface. The gap between the slot die apparatus and the tool surface istypically less than about two times that of the final sheetingthickness. Accordingly, the gap ranges from about 0.004 inches to 0.030inches when producing sheeting with a nominal thickness of 0.0025 inchesto 0.015 inches.

The resin is of a viscosity such that it flows, optionally with appliedvacuum, pressure, temperature, ultrasonic vibration, or mechanicalmeans, into the cavities in the molding surface. It is preferablyapplied in sufficient quantity that it substantially fills the cavities.In a preferred embodiment, the fluid resin is delivered at a rate suchthat the final land thickness of the sheeting (i.e., the thicknessexcluding that portion resulting from the replicated microstructure,1300 b in FIG. 4 c) is between 0.001 and 0.100 inches and preferablybetween 0.003 and 0.010 inches. With reference to FIG. 4 c, the surface1300 a of the resin (e.g., solidified) opposing the tool surface isgenerally smooth and planar. Alternatively, however, the resin may bedelivered in a manner such that the cube cavities alone are filled andthus the sheeting is substantially free of a land layer. In thisembodiment the cube corner elements are typically bonded to a film layerprior to removal from the tool surface.

In the case of extrusion of molten thermoplastic resins, the resin istypically initially provided in a solid pellet form and poured intohopper 2100 that continuously feeds the resin into a melt extruder 2000.Heat is typically supplied to the tool by passing over the drive roll1400A that is heated for example with circulating hot oil or by electricinduction to maintain a tool surface temperature above the softeningpoint of the polymer. Suitable cooling means such as spraying water ontothe extruded resin or tool, contacting the unstructured surface of thetool with cooling rolls, or direct impingement air jets provided byhigh-pressure blowers are provided after extrusion to sufficientlyharden the resin such that it may be removed from the tool.

In the case of polymerizable resins, the resin may be poured or pumpeddirectly into a dispenser that feeds slot die apparatus 1000. Forembodiments wherein the polymer resin is a reactive resin, the method ofmanufacturing the sheeting further comprises curing the resin in one ormore steps. For example the resin may be cured upon exposure to asuitable radiant energy source such as actinic radiation, ultravioletlight, visible light, etc. depending upon the nature of thepolymerizable resin to sufficiently harden the resin prior to removalfrom the tool. Combinations of cooling and curing may also be employed.

With reference to FIGS. 5-6, an exemplary slot die apparatus 1020 foruse in the invention comprises two portions, a first die portion 110 anda second die portion 115. The first and second die portions are joinedtogether at the die parting line 180 creating a slot cavity (not shown)having a rectangular slot orifice 181. Adjacent to the slot orifice 181and downstream of it relative to the direction of rotation 310 of roll1400 a, is a first die lip 120, also referred to herein as thedownstream lip. Adjacent to the slot orifice 181, and upstream of itrelative to the direction of rotation 310 of roll 1400 a, is a seconddie lip 170, also referred to herein as the upstream lip. These lips arebrought into close proximity to the continuously advancing moving tool1200 having a microstructured surface. The drive roll 1400 a is built toresist high die loading forces while maintaining overall roll surfacedeflection of less than 0.001 inches over the working face of the roll.

In the method of the present invention, the tool and/or moldable resinis advanced and thus provided such that the channels (i.e., primarygroove channels and/or structured channels and/or cube corner cavitychannel) of the tool are substantially parallel to the direction of theadvancing tool. In doing so the channels of the tool are preferablysubstantially normal to the slot orifice of the slot die apparatus. Thepresent inventor has discovered that providing the tool to thedispensing apparatus in a manner wherein the channels are perpendicularto the direction of the advancing tool results in poor tool filling.Poor tool filling is evident by the sheeting having an irregularappearance in plan view such as crossweb striations as depicted byportion A of FIG. 8 rather than being substantially free of suchirregularities as depicted by portion B of FIG. 8. Upon viewing thesheeting with a microscope, it is evident that portion A has substantialunfilled inclusions as depicted in FIG. 9 b, whereas the percentage ofunfilled inclusions in FIG. 9 a are less than 1%. Portion A of FIG. 8would also exhibit poor (if any) retroreflected brightness due to theseverity of the cube corner element defects.

Methods of machining laminae and forming a master tooling from laminaeare known, such as described in U.S. Pat. No. 6,257,860 (Lutrell etal.). For embodiments wherein the side grooves are substantially free ofskew and/or inclination, side grooves may be formed in a plurality ofstacked laminae, such as described in U.S. Pat. No. 6,257,860 (Lutrellet al.) and U.S. Pat. No. 6,159,407 (Krinke et al.). A preferred methodfor forming grooves on the edge of individual lamina (e.g., laminahaving side grooves comprising skew and/or inclination), assembling thelaminae, and replicating the microstructured surface of the assembledlaminae is described in U.S. Pat. No. 7,174,619; incorporated herein byreference. U.S. Pat. No. 7,174,619 was concurrently filed with U.S.patent application Ser. No. 60/452,605, to which the present applicationclaims priority. Such methods describe machining cube cornermicrostructures on an exposed edge surface portion of the lamina, (i.e.,working surface 16 with reference to FIG. 1) by forming a plurality ofV-shaped grooves with a groove-forming machine.

A lamina is a thin plate having length and height at least about 10times its thickness (preferably at least 100, 200, 300, 400, 500 timesits thickness). The lamina(e) are not limited to any particulardimensions. One of ordinary skill in the art appreciates the optimaldimensions of the lamina are related to the flexural stiffness of thelamina, buckling stiffness, and ease of handling. Furthermore, optimaldimensions may also be constrained by the optical requirements of thefinal design (e.g., cube corner structures). The lamina typically has athickness of less than 0.25 inches (6.35 mm) and preferably less than0.125 inches (3.175 mm). The thickness of the lamina is preferably lessthan about 0.020 inches (0.508 mm) and more preferably less than about0.010 inches (0.254 mm). Typically, the thickness of a lamina is atleast about 0.001 inches (0.0254 mm) and more preferably at least about0.003 inches (0.0762 mm). Such laminae range in length from about 1 inch(25.4 mm) to about 20 inches (5.08 cm) and are typically less than 6inches (15.24 cm). The height of a lamina typically ranges from about0.5 inches (12.7 mm) to about 3 inches (7.62 cm) and is more typicallyless than about 2 inches (5.08 cm).

In general, the lamina may be comprised of any substrate suitable forforming directly machined grooves on the edge. Suitable substratesmachine cleanly without burr formation, exhibit low ductility and lowgraininess and maintain dimensional accuracy after groove formation. Avariety of machinable plastics or metals may be utilized. Suitableplastics comprise thermoplastic or thermoset materials such as acrylicsor other materials. Machinable metals include aluminum, brass, copperelectroless nickel, and alloys thereof. Preferred metals includenon-ferrous metals. Suitable lamina material may be formed into sheetsby for example rolling casting chemical deposition, electro-depositionor forging. Preferred machining materials are typically chosen tominimize wear of the cutting tool during formation of the grooves. Othermaterials may also be suitable for lamina comprising other types ofmicrostructures.

The V-shaped grooves are preferably formed with a diamond-toolingmachine that is capable of forming each groove with fine precision.Moore Special Tool Company, Bridgeport, Conn.; Precitech, Keene, N.H.;and Aerotech Inc., Pittsburgh, Pa., manufacture suitable machines forsuch purpose. Such machines typically include a laserinterferometer-positioning device. A suitable precision rotary table iscommercially available from AA Gage (Sterling Heights, Mich.); whereas asuitable micro-interferometer is commercially available from ZygoCorporation (Middlefield, Conn.) and Wyko (Tucson, Ariz.) a division ofVeeco. The precision (i.e., point to point positioning) of themicrostructure (e.g., groove vertices spacing and groove depth) ispreferably at least as precise as +/−500 nm, more preferably at least asprecise as +/−250 nm and most preferably at least as precise as +/−100nm. The precision of the groove angle is at least as precise as +/−2 arcminutes (+/−0.033 degrees), more preferably at least as precise as +/−1arc minute (+/−0.017 degrees), even more preferably at least at preciseas +/−½ arc minute (+/−0.0083 degrees), and most preferably at least asprecise as +/−¼ arc minute (+/−0.0042 degrees) over the length of thecut (e.g., the thickness of the lamina). Further, the resolution (i.e.,ability of groove forming machine to detect current axis position) istypically at least about 10% of the precision. Hence, for a precision of+/−100 nm, the resolution is at least +/−10 nm. Over short distances(i.e., 10 adjacent parallel grooves), the precision is approximatelyequal to the resolution. In order to consistently form a plurality ofgrooves of such fine accuracy over duration of time, the temperature ofthe process is maintained within +/−0.1° C. and preferably within+/−0.01° C.

The diamond tools suitable for use are of high quality such as diamondtools that can be purchased from K&Y Diamond (Mooers, N.Y.) or ChardonTool (Chardon, Ohio). In particular, suitable diamond tools are scratchfree within 0.010 inches (0.254 mm) of the tip, as can be evaluated witha 2000× white light microscope. Typically, the tip of the diamond has aplanar portion ranging in size from about 0.00003 inches (0.000762 mm)to about 0.00005 inches (0.001270 mm). Further, the surface finish ofsuitable diamond tools preferably have a roughness average of less thanabout 3 nm and a peak to valley roughness of less than about 10 nm. Thesurface finish can be evaluated by forming a test cut in a machinablesubstrate and evaluating the test cut with a micro-interferometer, suchas can be purchased from Wyko (Tucson, Ariz.), a division of Veeco.

The method may employ a variety of dispensing apparatus for casting themoldable (e.g., fluid) resin onto the microstructured tool surface.Suitable dispensing apparatus include the slot die apparatus describedin U.S. Pat. No. 5,067,432 (Lippert) as well as slot die apparatuscommercially available from Extrusion Dies, Inc., Chippewa Falls, Wis.under the trade designations “Ultracoat” and “Ultraflex”. In someembodiments the method of the invention further employs the methods andapparatus described in U.S. Patent Publication No. 2004/0173920, titled“Method of Making Retroreflective Sheeting and Slot Die Apparatus”,incorporated herein by reference.

With reference to FIG. 6 an exemplary die comprises a downstream liphaving a total length of about 1.0 inch (e.g., 0.88 inches) having twosurface portions. The first surface portion extends from the leadingedge 130 to the trailing edge of the first surface portion 125 (i.e.,also the leading edge of the second surface portion and the line ofadjacency for the two surface portions) having a length 121 a of 0.41″and an angle 128 of 89.2° degrees to vertical as measuredcounterclockwise from the lip surface to an extrapolation of die partingline 180, as depicted in FIG. 6. The second surface portion extends fromthe trailing edge of the first surface portion 125 having a length 126 aof 0.47″ and an angle 122 of 86.8° degrees to vertical as measuredcounterclockwise from the lip surface to an extrapolation of die partingline 180, as depicted in FIG. 3. Further, the downstream lip comprisessufficient structural strength to resist flexing or deflection due tothe high pressure developed between the lip surface portions and theadvancing tool.

The method of the invention is suitable for use with any microstructuredesign, e.g., cube corner element design comprising primary groovesand/or microstructured channels and/or cube corner cavity channels aspreviously described. Since the tool is provided such that a majorportion of the channels are substantially parallel to the direction ofthe advancing tool, in the case of roll-goods the channels is alsosubstantially parallel to the longitudinal edges, i.e., maximumdimension, of the sheet.

The retroreflective sheeting preferably comprises an array of cubecorner microstructures wherein at least a portion and preferablysubstantially all the cube corner elements of the lamina(e) andretroreflective sheeting are full cubes that are not truncated. In oneaspect, the base of full cube elements are not triangular. In anotheraspect, the non-dihedral edges of full cube elements arecharacteristically not all in the same plane (i.e., not coplanar). Suchcube corner elements are preferably “preferred geometry (PG) cube cornerelements”. Full-cube and PG cube corner elements typically exhibit ahigher total light return in comparison to truncated cube cornerelements.

A PG cube corner element may be defined in the context of a structuredsurface of cube corner elements that extends along a reference plane.For the purposes of this application, a PG cube corner element means acube corner element that has at least one non-dihedral edge that: (1) isnonparallel to the reference plane; and (2) is substantially parallel toan adjacent non-dihedral edge of a neighboring cube corner element. Acube corner element whose three reflective faces comprise rectangles(inclusive of squares), rectangles, quadrilaterals, trapezoidspentagons, or hexagons are example of PG cube corner elements.“Reference plane” with respect to the definition of a PG cube cornerelement refers to a plane or other surface that approximates a plane inthe vicinity of a group of adjacent cube corner elements or othergeometric structures, the cube corner elements or geometric structuresbeing disposed along the plane. In the case of a single lamina, thegroup of adjacent cube corner elements consists of a single row or pairof rows. In the case of assembled laminae, the group of adjacent cubecorner elements includes the cube corner elements of a single lamina andthe adjacent contacting laminae. In the case of sheeting, the group ofadjacent cube corner elements generally covers an area that isdiscernible to the human eye (e.g., preferably at least 1 mm²) andpreferably the entire dimensions of the sheeting.

Suitable resin compositions for the retroreflective sheeting of thisinvention are preferably transparent materials that are dimensionallystable, durable, weatherable, and readily formable into the desiredconfiguration. Examples of suitable materials include acrylics, whichhave an index of refraction of about 1.5, such as Plexiglas brand resinmanufactured by Rohm and Haas Company; polycarbonates, which have anindex of refraction of about 1.59; reactive materials such as thermosetacrylates and epoxy acrylates; polyethylene based ionomers, such asthose marketed under the brand name of SURLYN by E. I. Dupont de Nemoursand Co., Inc.; (poly)ethylene-co-acrylic acid; polyesters;polyurethanes; and cellulose acetate butyrates. Polycarbonates areparticularly suitable because of their toughness and relatively highrefractive index, which generally contributes to improvedretroreflective performance over a wider range of entrance angles.Injection molding grade polycarbonate having a melt flow rate rangingfrom 17 g/10 min. to 24 g/10 min. (ASTM D1238 or ISO 1133-1991;condition 300/1.2) is typically preferred. These materials may alsoinclude dyes, colorants, pigments, UV stabilizers, or other additives.Although transparent synthetic resins are employed in the manufacture ofretroreflective sheeting, in the case of other microstructured articles,the synthetic resin may be opaque or translucent as well.

In the case of molten polymeric resins, the resin typically solidifiesas a function of sufficient cooling. For example, polycarbonatesufficiently cools upon reaching a temperature of about 240° F. orlower. Cooling can be achieved by any means including by spraying wateronto the extruded resin or tool, contacting the unstructured surface ofthe resin or tool with cooling rolls, or my means of direct impingementair jets provided by high-pressure blowers.

Other illustrative examples of materials suitable for forming the arrayof cube corner elements are reactive resin systems capable of beingcross-linked by a free radical polymerization mechanism by exposure toactinic radiation, for example, electron beam, ultraviolet light, orvisible light. Additionally, these materials may be polymerized bythermal means with the addition of a thermal initiator such as benzoylperoxide. Radiation-initiated cationically polymerizable resins also maybe used. Reactive resins suitable for forming the array of cube cornerelements may be blends of photoinitiator and at least one compoundbearing an acrylate group. Preferably the resin blend contains amonofunctional, a difunctional, or a polyfunctional compound to ensureformation of a cross-linked polymeric network upon irradiation.

Illustrative examples of resins that are capable of being polymerized bya free radical mechanism that can be used herein include acrylic-basedresins derived from epoxies, polyesters, polyethers, and urethanes,ethylenically unsaturated compounds, isocyanate derivatives having atleast one pendant acrylate group, epoxy resins other than acrylatedepoxies, and mixtures and combinations thereof. The term acrylate isused here to encompass both acrylates and methacrylates. U.S. Pat. No.4,576,850 (Martens) discloses examples of crosslinked resins that may beused in cube corner element arrays of the present invention.

The manufacture of the sheeting may include other optional manufacturingsteps prior to or subsequent to solidification of the sheeting. Forexample, the retroreflective sheeting can be manufactured as a layeredproduct by casting the cube-corner elements against a preformed film astaught in PCT publication No. WO 95/11464 and U.S. Pat. No. 3,684,348,or by laminating a preformed film to preformed cube-corner elements. Indoing so the individual cube-corner elements are interconnected by thepreformed film. Further, the elements and film are typically comprisedof different materials.

Alternatively or in addition thereto, specular reflective coating suchas a metallic coating can be placed on the backside of the cube-cornerelements. The metallic coating can be applied by known techniques suchas vapor depositing or chemically depositing a metal such as aluminum,silver, or nickel. A primer layer may be applied to the backside of thecube-corner elements to promote the adherence of the metallic coating.

In addition to or in lieu of a metallic coating, a seal film can beapplied to the backside of the cube-corner elements; see, for example,U.S. Pat. Nos. 4,025,159 McGrath) and 5,117,304 (Huang et al.). The sealfilm maintains an air interface at the backside of the cubes thatenables total internal reflection at the interface and inhibits theentry of contaminants such as soil and/or moisture. Further a separateoverlay film may be utilized on the viewing surface of the sheeting forimproved (e.g., outdoor) durability or to provide an image receptivesurface. Indicative of such outdoor durability is maintaining sufficientbrightness specifications such as called out in ASTM D49560-1a afterextended durations of weathering (e.g., 1 year, 3 years). Further theCAP Y whiteness is preferably greater than 30 before and afterweathering.

The retroreflective sheeting is useful for a variety of uses such astraffic signs, pavement markings, vehicle markings and personal safetyarticles, in view of its high retroreflected brightness. The coefficientof retroreflection, R_(A), may be measured according to US Federal TestMethod Standard 370 at −4° entrance, 0° orientation, 0.2° observation istypically at least 100 candelas per lux per square meter (CDL),preferably at least 300 CDL and more preferably at least 600 CDL.

Patents, patent applications, and publications disclosed herein arehereby incorporated by reference (in their entirety) as if individuallyincorporated. It is to be understood that the above description isintended to be illustrative, and not restrictive. Various modificationsand alterations of this invention will become apparent to those skilledin the art from the foregoing description without departing from thescope of this invention, and it should be understood that this inventionis not to be unduly limited to the illustrative embodiments set forthherein.

Examples 1-4

Retroreflective sheeting was prepared utilizing a slot die apparatusthat was substantially identical to a slot die apparatus commerciallyavailable from Extrusion Dies, Inc., Chippewa Falls, Wis. under thetrade designation “Ultracoat” with the exceptions that the downstreamdie lip was changed to incorporate the following specific features: (1)the horizontal length of the lip was changed from 0.478″ to 0.884″; (2)a section of the lip near the polycarbonate exit slot was thinned toprovide a hinge in the horizontal section of the lip, thereby allowingthe downstream portion of the lip to be adjusted in a vertical plane;(3) the polycarbonate contacting surface of this lip was machined toprovide two planar surfaces, the first surface being at an angle of 89.2degrees to vertical as measured counterclockwise from the tool surfaceto an extrapolation of the parting line of the die, and the secondsurface being 87.6 degrees to vertical counterclockwise as measuredcounterclockwise from the tool surface to an extrapolation of theparting line of the die; (4) die bolts were configured to push on theoutboard or trailing section of the die lip, such that adjustments ofthe bolts resulted in a vertical displacement of the trailing lip whilenot substantially changing the polycarbonate slot between the front andrear die lips; (5) the lip was built from P-20 tool steel such that withreference to FIG. 3 dimension 150 had a thickness of 0.33 inches,dimension 151 had a thickness of 0.26 inches, dimension 152 had athickness of 0.19 inches and angle 153 was 105° relative to referenceplane 2000.

In operation, the die was mounted such that the parting line slotorifice was positioned horizontally 0.050″ upstream of top dead centerrelative to a reference plane tangent to the roll at top dead center ina rigid framework having a die support beam and a jackscrew assembly formoving the die in a vertical plane so that it could be positioned at anydistance from a heated roller. The jackscrew assembly was equipped withactuating motors on both sides of the support beam that were mountedwith bolts to the die beam. The moving end of the jackscrew was threadedinto a load cell that was bolted to the main support structure in amanner that the net load developed by the molten resin interacting withthe die lips was transmitted through and sensed by the load cells. Theload cells were connected to suitable electronics that provided adigital display of these die forces. The die was attached to a singlescrew extruder.

Injection molding grade polycarbonate having a melt flow rate rangingfrom 17 g/10 min. to 24 g/10 min. (ASTM D1238 or ISO 1133-1991;condition 300/1.2) was dried for 4 hours in a 250° F. drying hopper. Thedried polycarbonate pellets were flood fed to the extruder inlet. Theextruder barrel zone temperatures were set at 475° F. for Zone 1, 535°F. for Zone 2, 550° F. for Zone 3, 565° F. for Zone 4 and 570° F. forZone 5. The gate exit zone temperature at the end of the extruder wasset at 575° F. The polymer melt temperature and pressure were measuredat the extruder gate using a melt thermocouple and a pressure proberespectively and are provided in TABLE 1. An adapter of 1.25 inchinternal diameter (I.D.) connected the extruder gate and the die. Thetemperature on the adapter was set at 560° F. The die body was dividedinto 16 temperature zones, each temperature zone having approximatelythe same area. The downstream lip included sequential zones 1-8, whereasthe upstream lip included sequential zones 9-16 with zones 1 and 16being adjacent to one another yet on opposite sides of the slot orifice.The zone temperatures were set at 575° F. for Zones 1, 2, 7, 8, 9 and10; 560° F. for Zones 3, 6, 11 and 14; 545° F. for Zones 4, 5, 12 and13; and 570° F. for Zones 15 and 16.

The die was initially positioned so that the downstream lip wasapproximately 10 mils from the microprismatic surface of a tool (as willsubsequently be described in greater detail) that consisted of theinverse of the desired microprismatic design of the retroreflectivesheeting. The vertical position of the trailing die lip 126 was adjustedby turning all the die bolts ⅜ of one turn resulting in the trailingedge 140 of the second surface portion being 0.004″ closer to the tool.The microprismatic surface was on a continuous metal belt that was setat the line speed provided in Table 1. The microprismatic surface of thetool was presented to the die by wrapping the belt around a continuouslydriven heated roller built to resist high die loading forces whilemaintaining overall roller surface deflection less than 1 mil over theworking face of the roller having a diameter of 30 inches. Heat wassupplied to the roller by a hot oil system with a set point of 495° F.The tool surface temperature was measure with a contact pyrometer onboth the left and right flat non-structured margins of the tooling asprovided in TABLE 1.

The tool was about 20 feet in the downweb direction by about 3 feet inthe crossweb direction. The tool included electroformed replicationsthat were derived from a master mold consisting of an assembly oflaminae having dimensions of about 2 inches by about 4 inches (i.e.,length of microprismatic surface of each lamina). The method ofmachining the lamina as well as the method of assembling and replicatingthe assembled laminae is described in previously cited U.S. Pat. No.7,174,619. The optical design formed in the lamina(e) is described inpreviously cited U.S. patent application Ser. No. 60/452,464. A primarygroove face extending substantially the full lamina length was formed oneach lamina. The primary groove face was oriented at roughly 35.49° tothe normal vector defined by the plane of the tool surface. Alternatingpairs of side grooves with included angles of substantially 75.226° and104.774° were formed in each lamina with a spacing of 0.005625 inches toproduce the remaining faces of the cube corner cavities. The symmetryaxes of the cubes corner cavities were canted about 6.03° in a planesubstantially parallel to reference plane 24. The side grooves wereformed substantially orthogonal to the primary groove face. The termsubstantially with regard to the side groove (i.e., included angles andorthogonal) refers to the side groove comprising a combination of ½angle errors, skew and inclination, each of which are less than 1° forthe purpose of introducing multiple non-orthogonality to improve theretroreflected divergence profile. Further details concerning groovescomprising skew and/or inclination is found in previously cited U.S.patent application Ser. No. 60/452,464, filed Mar. 6, 2003. Thecombination of ½ angle errors, skew and inclination is not believed toaffect the replication fidelity.

Since the tool surface is a negative replica of the assembled laminae,the tool comprises a plurality of substantially parallel cube cornercavity channels. The tool was provided to the slot die apparatus suchthe channel was normal to the slot orifice.

Photographs of retroreflective sheeting replicated from this tool aredepicted in FIG. 7. The horizontal dimensions of the cube cavities inFIG. 7 is 0.0075 mils, corresponding to the thickness of the individuallamina. The trapezoidal cube corner cavities of individual laminacorrespond to the vertical rows in FIG. 7.

Molten polycarbonate exited from the die orifice onto the microprismaticsurface of the tooling to form a continuous web of microreplicatedsheeting. The extruder output speed was adjusted to provide a 12 milnominal caliper of the sheeting. The die force was measured by the loadcells built into the supporting framework.

The belt and web continued from the curved surface of the roll into aflat free span zone and were then cooled by blowing air throughimpingement nozzles until a temperature of less than 240° F. wasreached. The web was then removed from the belt and wound into a roll ofretroreflective sheeting.

The replication fidelity of each retroreflective sheeting sample wasevaluated by taking four 3″×5″samples from each of four locations acrossthe web. Care was taken to ensure that the sixteen samples from eachcomparative example was taken from the same location. Each sample waslaid under a microscope (Measurescope MM-11) at a 10× magnification anda photograph (camera was a Javelin SmartCam) was taken of the samplewith the poorest replication. The photographs are depicted in FIGS. 7a-7 d. Poor replication would be evident by the presence of unfilledinclusions of the cube cavities that appear as black clusters at thecenter portion of each trapezoid, each trapezoid being the base edges ofthe cube corner element. The percentage of unfilled inclusions isapproximated by measuring the surface area of the unfilled inclusions inplan view. A “pass” rating refers to 1% or less unfilled inclusions,whereas a “fail” rating refers to greater than 1% unfilled inclusions.

TABLE 1 Example No. 1 2 3 4 Line Speed (fpm) 10 14 18 20 Extruder Speed(rpm) 7 9 11 13 Extruder Gate Melt 561 563 561 561 Temperature (° F.)Slot Pressure (psi) 886 730 785 661 Extruder Gate Melt 1789 1767 21642210 Pressure (psi) Tool Temperature (° F.) 425 405 400 398 Die Force(pli) 607 647 637 636 Replication Fidelity Pass Pass Pass Pass FIG. 5aFIG. 5b FIG. 5c FIG. 5d <1% loss <1% loss <1% loss <1% loss

Table 1 shows that each of Examples 1-4 exhibited good replicationfidelity.

Example 5 Comparative Example A

The same general procedure as described for Examples 1-4 was repeatedwith a different slot die apparatus wherein the total length of thedownstream lip was 0.501 inches and the downstream lip comprised asingle planar surface. The tool comprised cube corner cavity channelshaving cube cavities replicated from laminae forward canted by about9.74°. The width of the cube cavities was 7.5 mils corresponding to thethickness of a lamina and the side groove spacing was 5.0 mils. The cubepeak was centered between the side grooves and located 4.125 mils inplan view form the side of the lamina intersected by the primary groovesurface. All side grooves having an included angle of substantially 90°.The primary groove face was oriented at roughly 45° to the normal vectordefined by the plane of the tool surface.

In a first section of the tool, the tool was provided to the slot dieapparatus such that the cube cavity channels were orientated crosswebrelative to the direction of the advancing tool. Although thereplication was good at speeds less than 5 feet per minute, the toolfilling was poor at speeds greater than 5 feet per minute.

In a second section of the tool, the tool was provided to the slot dieapparatus such that the cube cavity channels were orientated downweb(i.e., channel parallel to the direction of the advancing tool) relativeto the direction of the advancing tool. The replication was good at arange of speeds up to about 28 feet per minute.

FIG. 8 depicts a photograph of retroreflective sheeting. Section A is aportion of the sheeting that was replicated at a rate of 11 feet/minutewith the channels of the tool orientated crossweb, whereas Section B isa portion of the sheeting that was replicated using the same conditionsexcept for the same tool being provided to the slot die orifice suchthat the channels of the tool were orientation downweb.

FIG. 9 a depicts a photograph of the retroreflective sheeting of SectionB, whereas FIG. 9 b depicts a photograph of Section A. As shown in thesephotographs Section A had substantial unfilled inclusions.

What is claimed is:
 1. A retroreflective sheeting comprising full cubemicrostructured elements having non-triangular reflective faces and anaverage height ranging from about 0.001 inches to 0.010 inches andincluding full cube microstructured elements having dihedral angleerrors, and having channels that are substantially parallel to alongitudinal peripheral edge of a roll of the sheeting, said channelsbeing selected from primary groove channels, structured channels, andcube cavity channels.
 2. The retroreflective sheeting of claim 1,wherein the full cube microstructured elements have reflective facesthat comprise rectangles, quadrilaterals, trapezoids, or pentagons. 3.The retroreflective sheeting of claim 1, wherein the sheeting includesrows of the full cube microstructured elements.
 4. The retroreflectivesheeting of claim 3, wherein the sheeting includes at least one channelfor each row of full cube microstructured elements.
 5. Theretroreflective sheeting of claim 1, wherein the sheeting has primarygroove channels parallel to structured channels.
 6. The retroreflectivesheeting of claim 5, wherein the structured channels are alternated withthe primary groove channels.
 7. The retroreflective sheeting of claim 1,wherein at least some of the channels include a substantially continuousplanar face.